"All U.S. government personnel are required to leave Mombasa," the embassy said in a statement Friday. "U.S. private citizens are not subject to the same restrictions, but should consider this information in their travel planning."

JUST WATCHED

East Africa's most advanced city

MUST WATCH

JUST WATCHED

Protecting Kenya's livestock owners

MUST WATCH

Protecting Kenya's livestock owners08:00

PLAY VIDEO

The warning did not elaborate on the threat or why it targeted the port city.

Kenyan forces invaded southern Somalia in October to battle Islamist militant group al-Shabaab, which it blamed for attacks and kidnappings of foreigners in the nation. The terror group has threatened Kenya with retaliatory attacks, saying it considers the forces' incursion an affront to Somalia's sovereignty.

Since the invasion, Kenya has been targeted by a series of grenade attacks blamed on the terror group or its supporters.

Al-Shabaab is linked to al Qaeda and has been designated a terrorist organization by the United States.